UNFOLD is an imaging method that speeds the image acquisition process in time-resolved applications such as magnetic resonance imaging (MRI) [1]. As its name suggests, UNFOLD (“Unaliasing by Fourier-encoding the Overlaps using the temporaL Dimension”) typically modulates aliased signals through the use of Fourier basis functions. UNFOLD forces aliased signals to behave in specific ways through time, so unwanted signals can be detected and removed. UNFOLD was originally introduced as a stand-alone method, and acceleration factors ranging from about 2 to as much as 8 have been presented [1]. More recently, it has been recognized that UNFOLD forms a valuable supplement to parallel imaging, in hybrid methods such as UNFOLD-SENSE, TSENSE and k-t SENSE [2-5]. UNFOLD can provide the coil-sensitivity information required by parallel imaging without the need for an extra calibration scan [3, 4], and it can reduce the artifact content as compared to parallel imaging alone [2-5]. Additional details regarding UNFOLD are provided in Madore U.S. Pat. No. 6,144,873, which is hereby incorporated by reference herein in its entirety.
Although UNFOLD has been successfully used in applications such as functional Magnetic Resonance Imaging (fMRI) and breath-held cardiac imaging, its use in the presence of breathing motion has proved more difficult. As noted by Di Bella et. al. [6] and by Ablitt et. al. [7], the respiration-related motion seen in myocardial perfusion studies may cause artifacts in UNFOLD-generated images. Artifacts in two-fold accelerated perfusion studies may consist of an edge-enhanced version of the anterior part of the thoracic cage, displaced along the phase-encoding direction from its true location, and sometimes partly obscuring the heart. Such artifacts can be suppressed to some extent by combining UNFOLD with parallel imaging. While UNFOLD has been shown to suppress artifacts caused by shortcomings in the parallel-imaging approach [2], the converse is equally true, and ghosting artifacts from UNFOLD can be suppressed by parallel imaging.
Ablitt et. al. [7] created a free-breathing version of UNFOLD for myocardial perfusion imaging, by separating data according to respiratory phase and applying UNFOLD separately to the different respiration-resolved bins. As discussed in [7], sorting the data into respiratory bins before applying UNFOLD filters should lead to a proper treatment of tissues moving due to respiration, but might be detrimental to signal from tissues undergoing other types of dynamic changes, such as contrast uptake.
In view of the foregoing, it would be desirable to provide improved methods for accelerated dynamic magnetic resonance imaging in the presence of breathing motion, and of other possible disruptions to the normal UNFOLD encoding scheme.